In such applications it is often desired for users of different optical sighting apparatuses to sight a common location. This is particularly true for urban environments where the risk of targeting inaccuracies causing unintended collateral damage is most acute. Furthermore, in an urban environment there is greatly increased risk of confused line-of-sight identification of a target's location, for example the wrong doorway, window, building, or street.
‘Triangulation’ is an ancient technique used in warfare and peace. Two or more observers take bearings on a target. From knowledge of their relative positions, the location of the target is easily determined. Both active and passive methods are used to determine the bearing. With active methods it is also generally possible to determine range based on time-of-flight (TOF), providing additional targeting measurements. For all methods of geometrical targeting, it is clearly essential to ensure that both observers are looking at the same target. If the target is obvious or the source of a unique identifier such as the sound or flash of a gunshot, this uncertainty may not apply. However, when using visual bearings in urban environments, there is often confusion over whether both/all observers are looking at the same target. Targeting often depends upon an exchange of verbal descriptions, which is highly susceptible to mistakes.
Current target designation systems often employ an active system wherein a laser is projected onto the target. The laser point may be detected by a monochromatic detector. However, active designation is not covert as the enemy may detect the laser indicating the fact that an object is being targeted, and worse still allowing the targeting laser to be located. Furthermore, the laser is relatively expensive to implement and increases the size of the equipment.
It is desirable that the equipment is portable. For example in a military application, this is a dominant constraining factor. Driven largely by consumer electronics, devices for determining position, heading, and range (including a global positioning satellite (GPS) receiver, microelectromechanical systems (MEMS) inertial measurement units (UMIs), magnetometers, and laser time-of-flight (TOF) rangefinders) have become small enough to be truly portable by an individual. Even with their power supplies, they represent minimal weight or size penalty to the user. While these devices are ideal for general outdoor navigation, when used alone, they are generally not sufficiently accurate for use in targeting. For example even with a limited standoff of 100 m, a 3 degree error which is typical for a MEMS device translates to a targeting error of 5 m, which is unacceptable in many military applications.